Recently, there has been an increasing demand for a reduction in the power consumption of electric products from the viewpoint of global environmental protection. In response to such a demand, not only reducing the power consumption of electric products in normal operation but also reducing the standby power consumption thereof has become a critical issue. Here, normal operation refers to a state in which all of the functions that an electric product has are operable. A state at the time of standby (standby state) refers to a state in which only the function or functions necessary for the electric product to return to normal operation are active.
A trigger input for making a transition from a standby state to a normal operating state is activated by, for example, pressing down the power button of the electric product or receiving a control signal (power-on signal or the like) from a remote controller. During a standby time, most load circuits are in a pause state. Consequently, power consumption decreases to an extremely small value, for example, one several tenth to one several hundredth, compared with power consumption during normal operation.
Examples of switching power supplies capable of reducing the power loss of a primary-side circuit at light load include a resonance-type switching power supply described in Patent Document 1 (Patent 003041842B). This resonance-type switching power supply includes, on the primary side of a transformer, a DC power source, a switching element connected to the DC power source, and a control circuit for controlling the switching frequency of the switching element. In power-saving mode, it is possible to lower the supply voltage supplied to the control circuit and reduce power loss by lowering the switching frequency.
Patent Document 2 (JP2003-033017A) describes a switching power supply capable of saving the power of a secondary-side circuit during standby time.
This switching power supply includes, on the secondary side of a transformer, an output voltage detecting circuit for detecting the output voltage of the transformer, a detecting/rectifying circuit for detecting and rectifying a high-frequency voltage output from a secondary winding, a voltage level determining circuit for determining an output voltage level of the detecting/rectifying circuit, and a superposing circuit for superimposing a predetermined voltage on an input terminal of the output voltage detecting circuit on the basis of a result of determination made by the voltage level determining circuit. The switching power supply includes, on the primary side of a transformer, a switching element, and a control circuit for controlling the operation of the switching element on the basis of the difference between a voltage value detected by the output voltage detecting circuit and a reference voltage value.
If the voltage level detected by the voltage level determining circuit is lower than the predetermined voltage (in the case of light load), the superposing circuit superimposes the predetermined voltage on the input terminal of the output voltage detecting circuit. By varying the input voltage of the output voltage detecting circuit by the predetermined voltage, the switching element is caused to perform burst switching operation to reduce secondary-side power loss.